Current transfer devices for electrical machines

ABSTRACT

Current transfer system for homopolar machines and other electrical machines having a direct current circuit passing from the stator to the rotor by way of a first brush and contact ring set and back from the rotor to the stator by way of a second brush and contact ring set, in which the brushes are on the stator in one set and on the rotor in the other set so that current flow between brushes and contact rings is in the same direction for both sets, preferably from the contact ring to the brush, thereby reducing brush wear.

United States Patent Wilkin et al.

[ Mar. 7, 1972 CURRENT TRANSFER DEVICES FOR ELECTRICAL MACHINESlnventors: Geoffrey Alan Wilkin; Ian Roderick Mc- Nab; Anthony DerekAppleton, all of Newcastle, England Assignee: International Research &Development Company Limited, Fossway, Newcastle upon Tyne, EnglandFiled: Feb. 5, 1971 Appl. No.: 112,876

Foreign Application Priority Data Feb. 5, 1970 Great Britain ..5,643/70U.S.Cl ..310/178, 310/219 Int. Cl. ..H02k 31/02 FieldofSearch..310/138,231,219,239,178,

[56] References Cited UNITED STATES PATENTS 1,706,369 3/1929 Williamson..310/219 3,073,979 1/1963 3,539,852 11/1970 Appleton et al. ..3 10/178Primary Examiner-D. F. Duggan Attorney-Kemon, Palmer & Estabrook [57]ABSTRACT Current transfer system for homopolar machines and otherelectrical machines having a direct current circuit passing from thestator to the rotor by way of a first brush and contact ring set andback from the rotor to the stator by way of a second brush and contactring set, in which the brushes are on the stator in one set and on therotor in the other set so that current flow between brushes and contactrings is in the same direction for both sets, preferably from thecontact ring to the brush, thereby reducing brush wear.

6 Claims, 2 Drawing Figures Patented March 7, 1972 3,648,088

2 Sheets-Sheet 2 III/[III l/l/ CURRENT TRANSFER DEVICES FOR ELECTRICALMACHINES This invention relates to current transfer in dynamo-electricmachines and is concerned particularly with machines in which currenttransfer takes place between brushes of electrically conducting materialand contact surfaces of electrically conducting material, there beingrelative rotation between the brushes and the contact surfaces, and thedirection of current flow between each brush and the associated contactsurface remains constant. Typical of such machines are homopolarmachines and synchronous electrical machines where, for example, adirect current is fed through brushes into slip rings on a rotor toprovide excitation for a field winding mounted on the rotor.

The problems presented by brush wear and electrical losses on homopolarmachines are acute because of the high currents which are transmitted bythe current transfer brushes. Brush wear can also be a problem in largeturbogenerators where high currents need to be transferred at highrotational speeds.

In accordance with the present invention there 'is provided adynamoelectric machine comprising a rotor and a stator, a direct currentcircuit extending from the stator to the rotor and back to the stator,first current transfer means in the said circuit for conveying currentfrom the stator to the rotor, and second current transfer means in saidcircuit for conveying current from the rotor to the stator, each of thecurrent transfer means comprising at least one brush and a contactsurface movable relative to and cooperating with said brush for thetransfer of current therebetween, the brush of the first transfer meansand the contact surface of the second transfer means being mounted onthe rotor and the brush ofthe second transfer means and the contactsurface of the first transfer means being mounted on the stator, wherebythe direction of current flow between the brush and the contact surfaceis the same for each of the current transfer means.

Preferably the direction of current flow is from the contact surfaceinto the brush, that is electrons pass from the brush to the contactsurface.

It is found that this arrangement of the brushes and contact surfacesresults in reduced brush wear.

The contact surfaces may be in the form of continuous slip rings or maybe composed of individual mutually insulated segments. The brushes maybe of carbon and may be arranged individually around a contact surfaceor form a substantially continuous ring. The brushes may be composed ofarrays of carbon fibers either in the form of individual brushes or as acontinuous ring. I

To allow for wear the contact surfaces may be frustoconical and be urgedunder a spring force in a direction transverse to the contact face ofthe brush, the movement being in a direction such that any gap formingbetween the brush and the surface is taken up.

The invention will now be described in more detail with the aid ofexamples illustrated in the accompanying drawings, in which:

FIG. 1 is a section of a homopolar electrical machine having a drum typerotor and current transfer means in accordance with the invention, and

FIG. 2 is a section ofa homopolar machine with a disc rotor and currenttransfer means in accordance with the invention.

The homopolar electrical machine shown in FIG. 1 has a stator whichcarries a first field coil composed of parts 11a and 11b and a secondfield coil composed of parts 12a and 12b. The field coils can be ofsuperconducting material in which case they are enclosed in a cryogenicenvelope in a conventional manner to maintain the low operatingtemperature which is required. In the case of superconducting coils nomagnetic core is required on the stator. Alternatively the field coilscan be of normal conducting material and in this case are disposed in amagnetic core on the stator 10 in conventional manner. The machine has adrum motor 13 which rotates within the stator on a shaft 14 and whichcarries on its periphery conducting paths 15 formed by a plurality ofindividual conductors extending axially of the rotor 13. Alternativelythe rotor may carry a single conducting path consisting -of a continuouscylindrical sleeve on the surface of the rotor.

of carbon brushes 16a mounted on the rotor for rotation with the rotor.The brushes 16a, each of which is composed of an assembly of carbonfibers, can form a complete ring extending substantially continuouslyaround the periphery of the rotor. The current transfer device 16 alsocomprises a frustoconical contact ring l6b which is mounted on thestator 10 between the parts 11a and 11b of the field coil. The contactring 16b is biassed in the direction of the arrows A by a spring (notshown) so that brush wear is compensated for by axial movement of thecontact ring.

The current "transfer device 17 comprises a number of brushes 17amounted on the stator 10 and engaging a contact surface composed of aring of contact segments 17b on the rotor. Each of the contact segments17b is connected to one end of a rotor conductor 15, whose other end isconnected to one of the brushes 16a. The contact segments 17b aremutually insulated from one another. The brushes 17a are located betweenthe two parts 12a and 12b of the field coil on the stator l0. 1

The brushes 17a on the stator are connected by leads 18 to one terminalof the machine while the contact ring 16b is connected by leads 19 tothe other terminal of the machine. Current flow thus proceeds from thisother terminal through the leads 19 to the contact ring 161) and thenceinto the brushes 16a. It then flows through the rotor conductors to thecontact segments 17b and from them into the brushes 17a and by way ofleads 18 to the external circuit. In each of the current transferdevices the current flow is from the contact surface into the brush andit is found that the electrical losses and brush wear rate areconsiderably improved over arrangements in which the direction ofcurrent flow is different in each current transfer device.

In an alternative arrangement the direction of current flow is reversedbutagain the direction of flow is the same in both current transferdevices. The arrangement is applicable to machines acting as motors orgenerators.

The contact segments 17b are shown as presenting a cylindrical contactsurface and the brushes 17a are biassed against this surface in a radialdirection in conventional manner. It is of course possible to use afrustoconical contact surface in the transfer device 17 as in the device16 and to bias the brushes axially against the contact surface. Whereasthe brushes 16a and 17a are composed of carbon fibers in the embodimentshown, and these fibers can be metal coated, it is also possible to useconventional carbon block brushes.

FIG. 2 shows the application of the invention to a homopolar machinewith a disc rotor. The machine has a stator 20 which supports asuperconducting field coil 21. A rotor support disc 22 is carried by ashaft 23 which is mounted in bearings (not shown) in the stator. Oneither side of the support disc 22 are conducting disc 24 and 25, forexample of copper, which are secured by studs 26. The outer periphery ofeach of the discs 24 and 25 has a flange which presents a contactsurface 24a and 25a, respectively, of frustoconical form towards therotor axis. These contact surfaces 240 and 250 are engaged by stationarybrushes 27 and 28, respectively, which are mounted on the statorstructure 20 and are connected to the external circuit by conductors 29and 30, respectively.

The inner regions of the discs 24 and 25 have cylindrical extensions 31and 32, respectively, which house brushes 33 and 34, respectively. Thebrushes 33 engage a fixed contact ring 35 which is connected to theexternal circuit by conductors 36. The brushes 34 engage a fixed contactring 37 which is connected to the external circuit by conductor 38.

The two discs 24 and 25 are connected in separate circuits each of whichincludes two current transfer devices, one for transferring current fromthe stator to the rotor and one for transferring the current from therotor to the stator and in each circuit the direction of current flow isthe same for each current transfer device. Thus one circuit runs fromthe contact ring to the brushes 33, through the disc 24 to the contactsurface 24a and thence to the brushes 27. The other circuit is from thecontact ring 37 to the brushes 34, through the disc 25 to the contactsurface 25a and thence to the brushes 28.

Each of the brushes 33 and 34 which rotate with the rotor may be acontinuous ring of carbon in solid or fiber form or may be composed ofseveral discrete brushes. The contact rings 35 and 37 may be urged in anaxial direction by springs (not shown) to compensate for brush wear.

The attitude of the interface between each set of brushes and theassociated contact surface is preferably arranged to follow thedirection of the magnetic field lines in that region in order to avoid avoltage being developed across the interface.

If the discs 24 and 25 are segmented as described in our U.S. Pat. No.3,497,739, each rotor disc will be divided up into separate mutuallyinsulated radial conducting paths each connected to a separate insulatedsegment on the outer flange and on the inner cylindrical extension. Thebrushes will be discrete brushes spaced by at least one segment widtharound the outer flange and inner cylindrical extension. The electricalconnections will be such as to connect selected conducting paths inseries as the rotor rotates. The contact rings 35 and 37 will also besegmented with insulation between the conducting segments.

The brushes may be held in position by conventional brush holders or iffiber brushes are used the arrays or bundles of fibers can be held incrimped tubes.

Whilst the invention has been described with particular reference tohomopolar machines it can also be applied to synchronous machines wheredirect current is transferred through slip-rings to a rotor winding.

Whilst carbon brushes have been described other brush materials can beused such as metal brushes, composite metalgraphite brushes or metalsuch as silver with molybdenum disulphide.

We claim:

1. A dynamoelectric machine comprising a rotor and a stator, a directcurrent circuit extending from the stator to the rotor and back to thestator, first current transfer means in the said circuit for conveyingcurrent from the stator to the rotor,

and second current transfer means in said circuit for convey-' ingcurrent from the rotor to the stator, each of the current transfer meanscomprising at least one brush and a contact surface movable relative toand cooperating with said brush for the transfer of currenttherebetween, the brush of the first transfer means and the contactsurface of the second transfer means being mounted on the rotor and thebrush of the second transfer means and the contact surface of the firsttransfer means being mounted on the stator, whereby the direction ofcurrent flow between the brush and the contact surface is the same foreach of the current transfer means.

2. A dynamoelectric machine as claimed in claim 1 wherein the directionof current flow in the direct current circuit is from the contactsurface into the brush.

3. A dynamoelectric machine as claimed in claim 1 in which each of thecontact surfaces comprises a plurality of mutually insulatedelectrically conductive segments.

4. A dynamoelectric machine as claimed in claim 1 in which each brushcomprises a block of carbon.

5. A dynamoelectric machine as claimed in claim 1 in which each brushcomprises an array of carbon fibers.

6. A dynamoelectric machine as claimed in claim 1 in which the contactsurfaces are frustoconical and are biassed against the brushes by forceapplying means acting in a direction transverse to the brushes.

1. A dynamoelectric machine comprising a rotor and a stator, a directcurrent circuit extending from the stator to the rotor and back to thestator, first current transfer means in the said circuit for conveyingcurrent from the stator to the rotor, and second current transfer meansin said circuit for conveying current from the rotor to the stator, eachof the current transfer means comprising at least one brush and acontact surface movable relative to and cooperating with said brush forthe transfer of current therebetween, the brush of the first transfermeans and the contact surface of the second transfer means being mountedon the rotor and the brush of the second transfer means and the contactsurface of the first transfer means being mounted on the stator, wherebythe direction of current flow between the brush and the contact surfaceis the same for each of the current transfer means.
 2. A dynamoelectricmachine as claimed in claim 1 wherein the direction of current flow inthe direct current circuit is from the contact surface into the brush.3. A dynamoelectric machine as claimed in claim 1 in which each of thecontact surfaces comprises a plurality of mutually insulatedelectrically conductive segments.
 4. A dynamoelectric machine as claimedin claim 1 in which each brush comprises a block of carbon.
 5. Adynamoelectric machine as claimed in claim 1 in which each brushcomprises an array of carbon fibers.
 6. A dynamoelectric machine asclaimed in claim 1 in which the contact surfaces are frustoconical andare biassed against the brushes by force applying means acting in adirection transverse to the brushes.